1. Field of the Invention
The present invention relates to a local oscillator signal divider and a low-noise converter employing this divider. In particular, the present invention relates to a power divider for a local oscillator signal used particularly for an LNB (Low-Noise Block Downconverter) of a satellite broadcasting receiver, for example, for dividing a local oscillator signal, and relates to a low-noise converter using this divider.
2. Description of the Background Art
FIG. 9 is a block diagram schematically showing a conventional and typical satellite broadcasting receiving system. Referring to FIG. 9, an LNB 100 is attached to an antenna 101 in a so-called outdoor unit. LNB 100 amplifies, with low noise, extremely weak radio wave in the frequency range of 12.2–12.7 GHz, and converts the frequency range to the range of 950–1450 MHz. The resultant signal of low noise and satisfactory level is then transmitted via a coaxial cable 103 to an indoor receiver 104.
Indoor receiver 104 includes a DBS (Direct Broadcast Satellite) tuner 105, an FM demodulator 106, a video and audio circuit 107, and an RF modulator 108. From signals supplied through coaxial cable 103, DBS tuner 105 selects a signal of a selected channel, and the selected signal is demodulated by FM demodulator 106. Video and audio circuit 107 accordingly supplies video and audio signals that are converted by RF modulator 108 into RF signals for a television receiver 109, and RF modulator 108 supplies the RF signals to TV receiver 109. LNB 100 may be the one with two inputs for polarized wave signals and one output (hereinafter two-input one-output LNB) as shown in FIG. 10 or the one with two inputs for polarized wave signals and two outputs hereinafter two-input two-output LNB) as shown in FIG. 11.
Referring to FIG. 10, the incoming signal in the range of 12.2–12.7 GHz received by antenna 101 shown in FIG. 9 is supplied to the two-input one-output LNB and divided into a right-handed polarized signal and a left-handed polarized signal by a phase plate 112 within an input waveguide 111. Right-handed and left-handed polarized signals are allocated alternately to broadcast programs of even-numbered channels and broadcast programs of odd-numbered channels in order to prevent adjacent channels from interfering with each other. The right-handed polarized signal and left-handed polarized signal are received respectively by antenna probes 113 and 114 to be supplied to an LNA (Low-Noise Amplifier) 130.
LNA 130 includes two amplifiers 131 and 132, a synthesizing circuit 133 and a gain amplifier 134. The right-handed signal is supplied to amplifier 131 while the left-handed signal is supplied to amplifier 132 and these signals are accordingly amplified, with low noise. Whether the left-handed signal or the right-handed signal is received, a power supply and switch control IC 140 switches on/off amplifiers 131 and 132. The right-handed signal output from amplifier 131 or the left-handed signal output from amplifier 132 is supplied from synthesizing circuit 133 via gain amplifier 134 to a BPF (Bandpass Filter) 135 where signal components (frequencies) in the image freqency band are eliminated. The received signal is then supplied to a mixer (MIX) 136.
A local oscillator circuit (Local) 137 generates a local oscillator signal of 11.25 GHz which is supplied to mixer 136. Mixer 136 mixes the received signal of 12.2–12.7 GHz with the local oscillator signal of 11.25 GHz in order to convert the received signal into an intermediate-frequency (IF) signal of 950–1450 MHz. The IF signal is then supplied to an IF amplifier (AMP.) 138. IF amplifier 138 having appropriate noise and gain characteristics amplifies the IF signal which is provided from an output terminal 139. A receiver, i.e., a television receiver is connected to output terminal 139 to receive only one of an odd-numbered channel and an even-numbered channel.
As discussed above, the two-input one-output LNB as shown in FIG. 10 is configured simply and at low cost, while only one of broadcast programs transmitted respectively by the right-handed signal and the left-handed signal can be received. In order to allow two broadcast programs transmitted respectively by a right-handed signal and a left-handed signal to be received simultaneously by different TV receivers, the two-input two-output LNB as shown in FIG. 11 should be employed.
Referring to FIG. 11, the incoming signal of 12.2–12.7 GHz received by antenna 101 in FIG. 9 is divided into a right-handed polarized signal and a left-handed polarized signal by a phase plate 112 within an input waveguide 111. The resultant right-handed polarized signal and left-handed polarized signal are respectively received by antenna probes 113 and 114 and supplied to LNAs 115 and 116 where the signals are amplified with low noise, and desired frequency components are supplied to BPFs 117 and 118 where signal components of the image frequency band are eliminated. The received signals are thereafter supplied to mixers 119 and 120.
A local oscillator circuit 121 generates a local oscillator signal of 11.25 GHz which is divided by a divider circuit 141 into two outputs with the one supplied via a blocking filter 129 to mixer 119 and the other one supplied via a blocking filter 128 to mixer 120. Blocking filters 129 and 128 block out any opposite polarization signals to prevent interference of received signals with each other and accordingly ensure isolation, which provides enhanced cross-polarization characteristics of these polarized signals. Mixers 119 and 120 mix received signals of 12.2–12.7 GHz with the local oscillator signal of 11.25 GHz so as to convert the received signals into intermediate frequency (IF) signals of 950–1450 MHz.
Respective IF signals output from mixers 119 and 120 are supplied to a switch circuit 122. Switch circuit 122 is controlled by a power supply and switch control IC 127 to supply respective outputs of mixers 119 and 120 to IF amplifiers 123 and 124. IF amplifiers 123 and 124 having appropriate noise and gain characteristics amplify respective IF signals, and resultant signals are supplied from output terminals 125 and 126 respectively. TV receivers are connected respectively to output terminals 125 and 126 to receive different broadcast programs simultaneously.
For such a downconverter with multiple inputs and multiple outputs as the two-input two-output LNB as shown in FIG. 11, it is necessary that required signals are transmitted respectively to a plurality of receivers. Then, the right-handed and left-handed polarized signals must be ready for output all the time and LNAs 115 and 116 and mixers 119 and 120 must always be in the driven state. Accordingly, the power of the local oscillator signal which is output from local oscillator 121 must be divided equally by divider circuit 141 into two outputs to be supplied respectively to two mixers 119 and 120.
Divider circuit 141 may be a Y-branch divider (Wilkinson coupler with the Y-shaped branch) as shown in FIG. 12. Y-branch divider 150 includes a port 1 connected to local oscillator 121, a port 2 connected to mixer 119 and a port 3 connected to mixer 120. For respective sections between port 1 and port 2 and between port 2 and port 3, a characteristic impedance of √{square root over (2)}·Zo and a length of λg/4 (λg: effective wavelength) are selected. A chip resistor with an impedance of 2Zo is connected between port 2 and port 3.
Y-branch divider 150 shown in FIG. 12 divides the local oscillator signal from local oscillator circuit 121 into two outputs to be supplied to two mixers 119 and 120, while lines respectively for right-handed and left-handed polarized signals extending from mixers 119 and 120 are connected via Y branch divider 150.
FIG. 13 shows pass characteristics from port 1 to port 2 as well as isolation characteristics from port 3 to port 2 of Y-branch divider 150. Similarly, FIG. 14 shows pass characteristics from port 1 to port 3 as well as isolation characteristics from port 2 to port 3. It is seen from FIGS. 13 and 14 that the isolation characteristics between port 2 and port 3 are deteriorated where the frequency is different from the center frequency. As Y-branch divider 150 has the narrow-band isolation characteristics and isolation is deteriorated at any frequency even slightly different from the frequency of 17.5 GHz, the right-handed and left-handed polarized signals each could leak to the opposite polarization which causes interference of the signals.
Then as shown in FIG. 11, the opposite polarization signals are blocked out by block filters 129 and 128 at the outputs of divider circuit 141 in order to ensure isolation of the right-handed and left-handed polarized signals. However, block filters 129 and 128 in a circuit for processing high frequencies are constituted by using microstrip lines, resulting in a disadvantage that the circuit size could increase. In order to overcome this disadvantage, any trap circuit is actually employed for the blocking. However, with regard to this trap circuit, trapped frequencies are significantly different depending on the length and there is a great influence of matching with respect to mixers 119 and 120. In order to achieve a stable circuit configuration by using the trap circuit, some technical knowledge is indispensable.